1. Field of the Invention
The present invention relates to a power supply device which supplies power to a circuit component, such as a semiconductor memory device or other memory devices, a method of the power supply device, and an image forming device including the power supply device.
2. Description of the Related Art
In various electronic devices, in order to prevent data loss in a memory caused by sudden power failure or trouble with a power supply, usually it is necessary to back up the memory in which the data are stored. Especially during data transmission, for example, by a facsimile machine or others, it is highly recommended to back up a memory integrated circuit (IC) which stores the received data or the data to be transmitted. In the related art, it is known that various techniques are used for this purpose, such as backup by using a super capacitor, or a power supply circuit technique using a single-cell battery to boost the power voltage.
In recent years, along with progress in semiconductor processing techniques, the integration degree of the integrated circuit (IC) keeps on increasing, and the inner structure of the integrated circuit (IC) is more and more miniaturized. Along with miniaturization of the integrated circuit (IC), the operating power voltage of semiconductors tends to be reduced in order to prevent damage inside the semiconductor that might occur were a high voltage to be applied to the semiconductor. On the other hand, along with an increasing scale of the electronic circuits and a rising operating frequency due to miniaturization of the IC, a consumption power current becomes large. In the future, along with more and more progress in the semiconductor processing techniques, it is expected that the inner structure of the integrated circuit (IC) will be more miniaturized, the operating power voltage will be lower, and the consumption power current will be larger. However, in the related art as described above, it is difficult to maintain a low operating power voltage and a large consumption power current, while ensuring to back up for a long time.
For example, in the backup technique using a super capacitor, in order to ensure back up for a long time, it is necessary to increase the capacity of the capacitor; due to this, the size of the capacitor increases. Hence, in order to obtain a long backup time period, the size of the electronic device becomes large. Further, since a capacitor having a large capacity is very expensive presently, using such a capacitor increases fabrication cost of the electronic device.
It is known that generally, the power supply circuit technique which uses a single-cell battery to boost the power voltage is capable of backup for a relatively long time without increasing the size of the electronic device compared to the backup technique using the super capacitor.
FIG. 8 is a block diagram illustrating a general circuit configuration for implementing the above power supply circuit technique.
As shown in FIG. 8, a power voltage V1 from a primary power supply 3, which is used as a usual power supply, or a power voltage V2 from an auxiliary power supply 4, is boosted through a DC-DC converter 2 to generate an operating voltage V0 to back up a device 1.
The usual power voltage V1 is used under usual operating conditions of the device 1, it is generated from an Alternating Current (AC) power supply, and is supplied by the primary power supply 3, namely, the usual power supply.
The auxiliary power voltage V2 is used under back operations of the device 1, it is generated from a Direct Current (DC) power supply, and is supplied by the auxiliary power supply 4. The auxiliary power supply 4 may be a Direct Current (DC) power supply. In addition, the Direct Current (DC) power supply may be a battery or capacitor, and its output voltage varies along with its discharging state.
Both of the usual power voltage V1 and the auxiliary power voltage V2 are lower than the operating voltage V0 of the device 1, and thus it is necessary for the DC-DC converter 2 to increase the usual power voltage V1 and the auxiliary power voltage V2.
In the circuit configuration of the power supply circuit technique, in order to respond to requirements of low operating voltage operations and increased currents of integrated circuits, it is required that the DC-DC converter 2 be able to work in a wide current range from a backup current to an operating current (for example, a few mA to a few amperes), and it is further required that the DC-DC converter 2 be capable of not only voltage step-up but also voltage step-down. When the device 1 is able to operate at a low voltage, usually, the usual power voltage V1 and the auxiliary power voltage V2 may be higher than the operating voltage V0 of the device 1, and in this case, it is required that the DC-DC converter 2 decrease the usual power voltage V1 and the auxiliary power voltage V2 to the operating voltage V0. As described above, the auxiliary power voltage V2 gradually decreases along with the discharging of the single-cell battery. As a result, when the auxiliary power voltage V2 becomes lower than the operating voltage V0, it is necessary to switch the operating mode of the DC-DC converter 2 to increase the auxiliary power voltage V2.
For example, Japanese Laid-Open Patent Application No. 9-65585 (hereinafter referred to as “reference 1”) discloses a battery backup power supply circuit capable of backup with a single-cell backup battery.
FIG. 9 is a circuit diagram illustrating an embodiment of the battery backup power supply circuit disclosed in reference 1.
FIG. 10 is a block diagram illustrating a functional configuration of the battery backup power supply circuit as shown in FIG. 9.
Note that the reference symbols in FIG. 10 correspond to the reference symbols assigned to the components of the battery backup power supply circuit shown in FIG. 9.
According to the configurations shown in FIG. 9 and FIG. 10, the battery backup power supply circuit is able to switch an input to the DC-DC converter between a usual operation mode and a backup operation mode, thereby, generating the power to a backup memory and a control circuit of the backup memory by the DC-DC converter.
However, since the battery backup power supply circuit disclosed in reference 1 switches the input to the DC-DC converter between the usual operation mode and the backup operation mode to implement voltage-step-up and voltage-step-down required in different operation modes, the battery backup power supply circuit is strongly dependent on the performance of the one DC-DC converter.
In the backup operations, in order to extend as much as possible the backup time period of the auxiliary power supply 4, it is necessary to use a power supply circuit of low power consumption and thus high efficiency. On the other hand, in the usual operations, it is necessary to use a power supply circuit able to conduct a large current in order to respond to requirements of an increased circuit scale due to the miniaturized ICs and increased operating frequencies.
However, in the power supply circuit disclosed in reference 1, the one DC-DC converter is commonly used in the backup operations and the usual operations, it is clear that there is a limit in optimizing the performance of the power supply circuit.
Generally, the DC-DC converter presently used has an efficiency change along with the magnitude of its current. When the DC-DC converter is used in a power supply circuit to support low operating voltage operations and increased currents of integrated circuits, as described above, it is required that the DC-DC converter be able to work in a wide current range. As a result, even when the DC-DC converter is optimized to have high efficiency in the backup operations involving a small current, the efficiency of the DC-DC converter declines in the usual operations involving a large current. This is not preferable from an energy-saving point of view.
On the other hand, even when the DC-DC converter is optimized to have high efficiency in the usual operations involving a large current, the efficiency of the DC-DC converter declines in the backup operations involving a small current, and the back-up time period becomes short. Among existing DC-DC converters, which are able to step-up and step-down voltages, support operations involving a large current, and have high efficiency, the available maximum current is only about 1 A; it is difficult to use these existing DC-DC converters to respond to the requirements of further lowered voltages and increased current and additional installation of memories caused by further progress in the semiconductor processing technology in the future. Further, the above DC-DC converters are also expensive, resulting in high cost.
As described above, in the related art, it is difficult to meet various conditions required by the devices to be backed up, while ensuring a long backup time period.